let a = ['a',1,true]

console.log(a)

var b;
let c ;
let d = {}
let e  = []
console.log(b)
console.log(c)
console.log(d.a)
console.log(e[2])
console.log(typeof undefined)
console.log(undefined instanceof Object)
console.log(undefined==='undefined')
console.log(typeof d.a==='undefined')
// console.log(f)           => error
console.log(typeof g)
// console.log(h instanceof undefined)  =>  error

let l = null
console.log(null instanceof Object)//false
console.log(l instanceof Object)//false
console.log(typeof l)//object

let p = undefined
console.log(p instanceof Object)//false
console.log(undefined instanceof Object)//false
console.log(typeof p)//undefined

console.log(null == undefined)//true
console.log(null == false)//false
if(!null){
    console.log('null') //=>null
}
if(!undefined){
    console.log('undefined') //=>null
}


let qq =  Boolean(0)
console.log(qq)
console.log(typeof qq)//object
console.log(qq === false) //=>false

// '0' => true
// 0 =>false
